Spondylosyndesis, or spinal fusion, is a surgical technique used to combine two or more vertebrae into a single, rigid working unit. This is typically achieved by introducing a supplementary bone tissue, such as an autograft or allograft, into the intervertebral space between two target vertebrae, at the location that is typically occupied by an intervertebral disc. The supplementary bone tissue is then used in conjunction with the patient's natural osteoblastic processes in order to grow bone or osseous tissue between the two or more target vertebrae, which acts to fuse them together into the desired rigid unit. This procedure is used primarily to eliminate pain that is caused by abnormal motion of one or both of the target vertebrae; pain relief occurs by immobilizing the vertebrae themselves and preventing the abnormal motion. Alternatively, surgically implantable synthetic intervertebral fusion cages or devices may be used to perform spinal fusion procedures.
Surgically implantable intervertebral fusion cages are well known in the art and have been actively used to perform spinal fusion procedures for many years. Their use became popularized during the mid 1990's with the introduction of the BAK Device from the Zimmer Inc., a specific intervertebral fusion cage that has been implanted worldwide more than any other intervertebral fusion cage system. The BAK system is a fenestrated, threaded, cylindrical, titanium alloy device that is capable of being implanted into a patient as described above through an anterior or posterior approach, and is indicated for cervical and lumbar spinal surgery. The BAK system typifies a spinal fusion cage in that it is a highly fenestrated, hollow structure that will fit between two vertebrae at the location of the intervertebral disc.
When in use, a bone graft implant or synthetic fusion cage may be filled with an orthopedic matrix containing additional fusion-promoting material (FPM), for example including but not limited to calcium hydroxyapatite, bone morphogenic protein (BMP), demineralized bone matrix, collagen bone graft matrix (e.g. Formagraft®) and stem cell material (e.g. Osteocel®) or other fusion-promoting substance placed within the spaces of the implant. The implant is then implanted into a patient at the desired location along that patient's spine where it will serve to promote bone growth and, ultimately, fusion of the two target vertebrae. The fenestrations present in a typical intervertebral fusion cage allow the supplementary bone tissue to partially escape from the hollow interior of the cage and make sufficient contact with the target vertebrae, thereby promoting fusion of the target vertebrae through the fusion cage itself. Unfortunately, the fenestrations, which are essential for the proper functioning of the intervertebral fusion cages, also present a major problem during surgical implantation of the cage in that they are poor retainers of the FPM inside of the fusion cage during implantation because they cannot, and do not, retain the supplementary bone material inside of the fusion cage during implantation. The supplementary bone material typically used in conjunction with an intervertebral fusion cage may vary in viscosity, however one example of a FPM typically used in conjunction with an intervertebral fusion cage is a viscous liquid that does not move as freely as water, however it is sufficiently liquid in form so as to readily move from, or leak out of, the interior chamber of the fusion cage during implantation. Since this material is intended to promote the formation of bony tissue inside of a patient's body, a leak of this material out of the fusion cage during implantation can lead to the creation of bony structures or osseous tissue at a location away from the surgical site and in locations that can cause severe medical complications for the patient, such as heterotopic bone formation, nerve compression, failure to properly heal the fusion area between the two target vertebrae, and numerous other complications.
It would be advantageous to provide tools suitable for use with a surgically implantable intervertebral fusion cage that is adapted to and/or capable of retaining the FPM inside of the cage during implantation and/or thereafter, until such time as it is desirous for the FPM to come into contact with the target vertebrae, thereby preventing the material from flowing from, or moving out of, the interior of the fusion cage during implantation and thereby significantly reducing, if not eliminating, the risks noted above.